This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-361594, filed Dec. 20, 1999; and No. 2000-300991, filed Sep. 29, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a nonaqueous electrolyte secondary battery, a carbon material for a negative electrode used for preparing a negative electrode of a battery such as a nonaqueous electrolyte secondary battery, and a method of manufacturing a carbon material for the negative electrode.
In recent years, various portable electronic appliances are being propagated in accordance with a rapid progress in the miniaturization technology of an electronic equipment. Miniaturization is also required for a battery used as a power source of these portable electronic appliances, and a nonaqueous electrolyte secondary battery having a high energy density attracts attentions.
A nonaqueous electrolyte secondary battery using metal lithium as the negative electrode active material has a very high energy density. However, the secondary battery of this type is short in the battery life because a tree branch-like crystals called dendrite are precipitated on the negative electrode in the charging step. The secondary battery of this type also leaves room for further improvement in safety because the dendrite grows to reach the positive electrode so as to bring about a problem such as an internal short circuit.
Under the circumstances, it is attempted to use a lithium alloy, a carbon material, an amorphous chalcogen compound in place of the lithium metal as the negative electrode active material. However, when it comes to the negative electrode containing a lithium alloy, the lithium alloy tends to be finely pulverized in accordance with progress of the charge-discharge cycles, with the result that the secondary battery is rendered poor in the cycle life. Also, when it comes to the negative electrode containing an amorphous chalcogen compound, an irreversible reaction tends to take place in the initial charging step, leading to a problem that the initial charging efficiency is low. Such being the situation, a carbon material capable of ensuring a safety and a long cycle life of the secondary battery is put to a practical use substantially exclusively as the negative electrode active material of the nonaqueous electrolyte secondary battery.
It was customary in the past to use as a carbon material in a nonaqueous electrolyte secondary battery a carbonized material or a graphitized material obtained by subjecting a carbon precursor such as pitch, coke, and polymer to a heat treatment under an inert gas atmosphere as well as a natural graphite, synthetic graphite and a low temperature calcined carbon.
However, since the capacity of the carbon material used as a negative electrode material is small compared with, for example, lithium metal and a lithium alloy, the nonaqueous electrolyte secondary battery comprising a negative electrode containing a carbon material gives rise to a problem that it is impossible to obtain a high discharge capacity.
Japanese Patent Disclosure (Kokai) No. 5-28996 recites a secondary battery comprising at least a positive electrode active material, a negative electrode active material, and an organic electrolyte, characterized in that a natural graphite subjected to a heat treatment at 400 to 800xc2x0 C. in the presence of an atmosphere consisting of an inert gas or in the vacuum before use is used singly or in combination with another material as a negative electrode material.
On the other hand, Japanese Patent Disclosure No. 6-290781 discloses a lithium secondary battery in which a natural graphite is used as a negative electrode material capable of absorbing-desorbing lithium ions, characterized in that said natural graphite is subjected to a heat treatment under temperatures not lower than 1800xc2x0 C. in the presence of an atmosphere consisting of an inert gas.
Further, Japanese Patent Disclosure No. 9-55204 discloses a method of manufacturing a lithium ion secondary battery comprising an anode containing carbon capable of reversibly inserting lithium, wherein the carbon is heated in a sufficiently long time and under a sufficient high temperature in the presence of an atmosphere containing O2 before the battery is assembled so as to selectively oxidize and gasify the undesirable highly reactive carbon atoms, thereby removing the undesirable highly reactive carbon atoms.
Further, a method of removing the impurities by applying a heat treatment to a carbon material is described in xe2x80x9cJournal of Power Sources, Vol. 76, pp 180-185, 1998xe2x80x9d.
On the other hand, claim 1 of Japanese Patent Disclosure No. 10-40914 recites a nonaqueous secondary battery, comprising a negative electrode containing as a negative electrode active material graphite particles having an amorphous carbon attached to the surface, a positive electrode containing a chalcogen compound containing lithium as a positive electrode active material, and a nonaqueous ionic conductor, characterized in that the negative electrode active material is prepared by subjecting graphite particles to an oxidizing treatment, followed by attaching an amorphous carbon to the surfaces of the graphite particles.
Also, claim 1 of Japanese Patent Disclosure No. 10-214615 discloses a nonaqueous secondary battery, comprising a negative electrode, a positive electrode containing a chalcogen compound, which contains lithium, as a positive electrode active material, and a nonaqueous ionic conductor, characterized in that the negative electrode contains as a negative electrode active material a carbon material prepared by attaching an amorphous carbon to the surfaces of graphite particles subjected to an oxidizing treatment with potassium permanganate.
However, any of the secondary batteries disclosed in the six publications referred to above failed to exhibit a sufficient discharge capacity.
An object of the present invention is to provide a nonaqueous electrolyte secondary battery having a high capacity and excellent in the cycle life characteristics.
Another object of the present invention is to provide a carbon material for a negative electrode capable of increasing the capacity of a nonaqueous electrolyte secondary battery and a method of manufacturing the particular carbon material for a negative electrode.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a positive electrode, a nonaqueous electrolyte, and a negative electrode containing a carbon material having an immersion heat ratio (xcex94Hin/xcex94Hih) defined by formula (1):
1.2xe2x89xa6xcex94Hin/xcex94Hihxe2x89xa62xe2x80x83xe2x80x83(1)
where xcex94Hih denotes the immersion heat for n-heptane of the carbon material, and xcex94Hin denotes the immersion heat for 1-nitropropane of the carbon material.
According to a second aspect of the present invention, there is provided a carbon material for a negative electrode, having an immersion heat ratio (xcex94Hin/xcex94Hih) defined by formula (1):
1.2xe2x89xa6xcex94Hin/xcex94Hihxe2x89xa62xe2x80x83xe2x80x83(1)
where xcex94Hih denotes the immersion heat for n-heptane of the carbon material, and xcex94Hin denotes the immersion heat for 1-nitropropane of the carbon material.
According to a third aspect of the present invention, there is provided a first method of manufacturing a carbon material for a negative electrode, comprising the step of applying a heat treatment to a carbonaceous material containing at least one material selected from the group comprising a carbonized material and a graphitized material under a gaseous atmosphere selected from the group consisting of a first gaseous atmosphere containing at least 10% by volume of CO2, a second gaseous atmosphere containing at least 1% by volume of H2O, and a third gaseous atmosphere containing at least 10% by volume of CO2 and at least 1% by volume of H2O.
According to a fourth aspect of the present invention, there is provided a second method of manufacturing a carbon material for a negative electrode, comprising the step of applying a heat treatment to a carbon precursor under a gaseous atmosphere selected from the group consisting of a first gaseous atmosphere containing at least 10% by volume of CO2, a second gaseous atmosphere containing at least 1% by volume of H2O, and a third gaseous atmosphere containing at least 10% by volume of CO2 and at least 1% by volume of H2O so as to carbonize or graphitize the carbon precursor.
According to a fifth aspect of the present invention, there is provided a third method of manufacturing a carbon material for a negative electrode, comprising the step of applying a heat treatment to a carbonaceous material containing at least one material selected from the group comprising a carbonized material and a graphitized material under an atmosphere containing at least one of a gas of an inorganic acid and a gas of an organic acid.
According to a sixth aspect of the present invention, there is provided a fourth method of manufacturing a carbon material for a negative electrode, comprising the step of:
applying a heat treatment to a carbonaceous material containing at least one material selected from the group comprising the carbonized material and the graphitized material under a gaseous atmosphere selected from the group consisting of a first gaseous atmosphere containing at least 10% by volume of CO2, a second gaseous atmosphere containing at least 1% by volume of H2O and a third gaseous atmosphere containing at least 10% by volume of CO2 and at least 1% by volume of H2O; and
bringing the carbonaceous material into contact with a gaseous acid.
Further, according to a seventh aspect of the present invention, there is provided a fifth method of manufacturing a carbon material for a negative electrode, comprising the step of:
applying a heat treatment to a carbon precursor under a gaseous atmosphere selected from the group consisting of a first gaseous atmosphere containing at least 10% by volume of CO2, a second gaseous atmosphere containing at least 1% by volume of H2O and a third gaseous atmosphere containing at least 10% by volume of CO2 and at least 1% by volume of H2O so as to carbonize or graphitize the carbon precursor; and
bringing the carbon precursor into contact with a gaseous acid.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.